In every nuclear reactor there must be arranged a quantity of fissionable material as a fuel and other materials as a moderator such that a "chain" reaction is achieved. The mass of fissionable material is termed a "critical mass". In order that the nuclear reactor can be operated over an appreciable period of time there must be included an excess of fuel above the critical mass, with this excess representing the fuel that will be consumed during operation of the reactor. As this extra fuel makes available a quantity of neutrons greater than the quantity necessary to perpetuate a controlled chain reaction, these excess neutrons must be absorbed in some manner so that an uncontrolled reaction does not result. The inherent ability of the excess fuel to produce these excess neutrons is generally referred to as "excess reactivity".
In the field of liquid (usually water) moderated nuclear reactors, such as pressurized water reactors (PWR), one technique for the control of reactivity is to produce an initial "spectral shift" which has the effect of increasing the epithermal (low reactivity) part of the neutron spectrum at the expense of the thermal (high reactivity) part. This results in production of fewer thermal neutrons and decreased fission. Then, as fission decreases during extended reactor operation, a reverse shift back to the thermal part of the neutron spectrum is undertaken.
There have been numerous systems developed to achieve this spectral shift. One such system is described in U.S. Pat. No. 3,081,246 issued to M. C. Edlund on Mar. 12, 1963. This system utilized the control of the ratios of heavy and light water used as moderator (and coolant) in the rector during operation. More recently various mechanical systems have been developed to effect the volumetric ratio between the fuel and the moderator to achieve the spectral shift concept. Typical of these systems are described in U.S. Pat. Nos.: 4,657,726 issued to D. B. Lancaster, et al., on Apr. 14, 1987; 5,683,103 issued to R. G. Lott, et al., on July 28, 1987; 4,683,116 issued to H. M. Ferrari, et al., on July 28, 1987; 4,687,620 issued to A. J. Impink, Jr., on Aug. 18, 1987; 4,687,621 issued to H. M. Ferrari on Aug. 19, 1987; 4,687,627 issued to J. F. Wilson, et al., on Aug. 18, 1987; 4,710,340 issued to W. J. Dollard, et al., on Dec. 1, 1987; and 4,716,007 issued to W. R. Carlson, et al., on Dec. 29, 1987.
In all but the '621 of the "mechanical regulation" patents, there are a plurality of "displacer rods" that can be moved within the reactor. Initially these displacer rods are fully inserted so as to displace a portion of the water within the reactor. As reactor operation proceeds, these rods are removed so as to add a higher proportion of water and thus more moderation as the fuel is consumed to achieve the spectral shift. Generally these displacer rods are grouped for a single fuel element, or a group of elements, so that a single mechanism can be used to accomplish the removal. As such, groups of displacer rods are attached to a "spider", with that spider being moved axially in the reactor with a suitable drive means (usually a motor-gear means). In order that this removal can be effectively achieved, each displacer rod must be provided with guides to prevent non-axial movement. For a given reactor, many groups of displacer rods are used, and it may be desirable that removal of one group is at different times relative to another group. This removal must be accomplished without deleteriously affecting temperature and neutron flux gradients within the reactor. Thus, very complex mechanical means and controls are required to accomplish regulation of appropriate moderation of the nuclear reaction with the displacer rods of the prior art.
In the '621 patent, these displacer rods contain burnable neutron poison material. Provision is made, via rupture elements, to permit gradual dissolution of the burnable poison material, with this material entering into the coolant and thus the moderator. This poison provides control of the excess reactivity. As the poison burns, together with the burn-up of the fuel, the reactor continues to be controlled.
Of course, in any of the reactor designs, there are normal control rods that regulate the level of operation of the nuclear reactor.
Accordingly, it is an object of the present invention to provide a method and apparatus for increasing the fuel efficiency of a nuclear reactor without adding elaborate mechanical and electrical controls.
It is another object of the present invention to provide this increased fuel efficiency using substantially conventional reactor construction without the complexity of movement of displacer rods as called for in the prior art reactor designs.
A further object of the present invention is to provide displacer rods for a nuclear reactor to initially provide proper moderation for any excess reactivity of a pressurized water reactor, with these displacer rods having a selected dissolution or volatilization/sublimation rate, whereby the volume of the displacer rods is gradually decreased as the nuclear fuel is burned so as to control (increase) the volume of the moderator during reactor operation.
Still another object of the present invention is to provide a spectral shift nuclear reactor in which displacer rods formed of a sacrificial material provide for a change in the fuel-to-moderator ratio without mechanical movement and take into account temperature and neutron flux gradients within the reactor.
Another object of the present invention is to provide controlled dissolution of displacer rods in a pressurized water reactor, with the product of that dissolution having no effect upon the nuclear characteristics of the reactor.
These and other objects of the present invention will become apparent upon a consideration of the drawings that follow together with a detailed description of the invention.